Poxviruses are double stranded DNA viruses that replicate in the cytoplasm of infected cells. Poxviruses encode enzymes needed to replicate their DNA, transcribe viral genes and process viral mRNA. Viral genes can be sorted into three classes that differ in their time of expression, promoter utilization and transcription initiation factors. Early genes are transcribed in the virus core immediately after infection. Early genes are the only class that responds to sequence dependent transcription termination. Early gene transcription termination requires the signal UUUUUNU in the mRNA, NPH I, a ssDNA dependent ATPase, VTF, the viral termination factor in the virion RNA polymerase that has a unique subunit, Rap94, the product of gene H4L. Recently, we showed that addition of an RNA oligonucleotide that has the sequence UUUUUUUUU stimulated premature transcription termination in vitro and in isolated virus cores. Premature termination was not observed when RNA possessing a mutant or chemically modified termination signal was added, demonstrating specificity. Importantly, stimulation of premature termination was independent of the template DNA sequence. Thus all early gene transcription termination is sensitive to the addition of a UUUUUNU containing oligonucleotide. We propose to exploit this observation and identify chimeric RNA oligonucleotides that inhibit poxvirus replication through the stimulation of premature transcription termination. We will synthesize and evaluate the ability of selected chemically modified oligonucleotides to stimulate premature termination in vitro and in isolated virus cores. Active oligonucleotides will be tested for their ability to inhibit virus gene expression and replication in tissue culture. Finally, the most active chimeric oligonucleotides will be tested for their ability to inhibit ectromelia virus (mousepox) replication in BALB/c mice. These studies will permit us to determine whether safe effective anti-poxvirus drugs can be synthesized based on their ability to inhibit virus replication through inhibition of early mRNA synthesis.